1. Field of the Invention
The invention relates to environmentally safe, non-mercury electronic switches. More particularly, the invention pertains to electronic tilt switches utilizing an electrically conductive sphere and having good resistance to physical damage of the sphere caused by electric arcing.
2. Description of the Related Art
The present invention relates to electronic devices, such as tilt switches that are alternately electrically turned "on" and "off" based on the position of a conductive sphere. A typical tilt switch comprises a hollow, enclosed housing or capsule having electrodes within the housing and extending through and outside the housing. Conventionally, a conductive material either in the form of a fluid, a weight or both is placed within the hollow housing. When using a fluid conductor, the housing is aligned in an "on" position when the conductive fluid electrically connects each of the electrodes, allowing the flow of electricity. The housing is aligned in an "off" position when the conductive fluid in the housing is tilted and does not electrically connect the electrodes, preventing the flow of electricity.
Traditionally, liquid mercury has been the preferred conductive material because it remains fluid at a wide range of temperatures, including room temperature. Also, mercury has been desirable because it has high surface tension, does not wet many surfaces that it contacts, and generally does not become damaged by contact with the electrodes. However, mercury, its vapor and its oxidized products are extremely toxic. Exposure to mercury has been known to cause disorders such as psychiatric problems in humans and disruption the endocrine systems of humans and animals. This has created a need in the art for a non-toxic alternative.
One commonly employed alternative has been to substitute a moveable conductive sphere for the liquid mercury. A typical tilt switches using a conductive sphere generally requires that the spherical conductor roll along a predefined path to alternately move from an electrically conductive position to an electrically nonconductive position. The conductive sphere is generally free moving and exhibits low rolling friction on an inclined surface. U.S. Pat. No. 5,136,127 teaches a tilt switch having at least one free moving spherical weight positioned in a housing that moves to an operating position when the weight is biased by gravity by inclination of the housing. However, conductive metal spheres have certain disadvantages. For example, conductive spheres are especially vulnerable to surface damage caused by electric arcing. When electrical current is made or broken by the spherical conductor moving into contact or out of contact with stationary conductors, or electrodes, it is common for arcing to occur. This arcing can create pitting on the surface of the sphere that may corrode the sphere and interfere with the smooth rolling of the sphere during later cycles of its operation.
U.S. Pat. No. 5,136,127 offers one solution to this problem, providing a tilt switch having first and second electrically conductive end caps, with each end cap having a non-conductive support edge, and a conductive sphere disposed in a space between the end caps. The sphere allows for the flow of electricity when positioned in contact with each of the conductive end caps, and will cut off electricity when the sphere is tilted out of electrical contact with the second end cap, contacting the non-conductive support edge of the second end cap. Any pitting of the sphere caused by arcing will not significantly interfere with rolling of the sphere because the sphere does not roll into and out of electrical contact with the conductive end caps, but tilts into and out of electrical contact with the end caps. However, pitting still interferes with electrical contact.
U.S. Pat. No. 5,209,343 offers another solution wherein a housing encapsulating the weight is filled with an inert atmosphere that will not react with the material of the weight to prevent pitting or other corrosion from forming on the weight that might adversely effect both the ability of the weight to move and the surface conductivity of the weight. Similarly, U.S. Pat. No. 5,543,767 teaches a tiltable switch wherein electrical arcing is prevented by filling a housing with an arc quenching gas such as argon or hydrogen, or by forming a vacuum in the housing.
One common problem with each of these existing tilt switches is the cost and complexity of manufacture and assembly which is often prohibitive in applications that require inexpensive switches to permit the application to be economically justifiable. The present invention offers a solution to the disadvantages of the related art.
The present invention provides a tiltable electrical device comprising a housing having electrodes extending from inside, through and outside the housing, at least one free moving electrically conductive sphere inside the housing and a lubricant at least partially filling the housing. The lubricant prevents electrical arcing from pitting the conductive sphere, eliminating corrosion of the sphere and maintaining its ability to freely roll and make good electrical contact. This device is an efficient and non-toxic replacement for conventional mercury switches. Further, the process provided by the invention requires no complex manufacturing steps and is a less expensive alternative than switches of the prior art.